This invention relates to musical instruments, particularly electronic organs, and especially to apparatus for producing pulsato by acoustic devices. An example of such apparatus is shown and described in U.S. Pat. No. 3,245,284 dated Apr. 12, 1966, issued to Donald J. Leslie.
During the course of a musical composition, the organist often desires to change the type of pulsato to be added. For example, he may wish to select acoustic pulsato, electronic pulsato or no pulsato. The selection is customarily made by operating organ stops or switches. In some applications, the switches merely shift the electrical signal from rotary pulsato apparatus to a non-rotary speaker. Alternatively, the acoustic pulsato apparatus, which includes moving parts such as rotors, may be stopped and started by the operation of switches. When acoustic pulsato is not desired, it may yet be desired slowly to shift the sound source at a rate well below pulsato frequencies, for example, at about two-thirds of a cycle per second in order to produce what has come to be defined as "chorus." A system for achieving the latter effect is described in the aforementioned U.S. Pat. No. 3,245,284 and includes two motors cooperatively arranged to drive a pulsato rotor either at full pulsato speed, or at a much slower rate to produce chorus.
In the patented system, the main motor for driving the rotor at full pulsato speed is arranged so that it is always coupled to the rotor by a pulley arrangement operated from one end of the motor shaft.
A secondary motor has an axially floating rotor that is spring biased normally to be out of alignment with its stator. When the secondary motor is energized, the floating rotor pulls into alignment by solenoid action. This movement is utilized to provide a releasable friction coupling between the shaft of the secondary motor and a friction wheel carried at the remote end of the main motor. With the secondary motor energized and the main motor de-energized, the acoustic rotor is thus driven at a slower chorus rate. While the patented arrangement produces satisfactory results and has enjoyed a long period of commerical acceptance, it has the disadvantage that when the main drive motor is turned off following operation at full pulsato speed, the pulsato apparatus coasts to a stop without any braking effect; the resulting rather long coasting period results in certain undesirable droning sounds. The patent teaches that this difficulty can be overcome by utilizing a time delay device to cause the slow speed secondary motor to be energized for a period of a few seconds during which its shaft is displaced so as to engage and drive the main motor at slow speed, after which the time delay circuitry shuts off the slow drive motor, whereupon the pulsato rotor comes to a halt. While this system produces satisfactory results, it does utilize relatively complicated time delay circuitry and switching equipment which, in the interest of reducing the cost of manufacture and the risk of failure, is desirably eliminated.
It is the primary object of the present invention to provide by mechanical means alone, that is, without the requirement of electrical delay devices, an inexpensive motor drive system for driving a pulsato rotor at either pulsato speed or slow chorus speed, which also provides automatic braking from full pulsato speed to either slow speed or to a completely stopped condition.